DESCRIPTION: (Applicant's Description) Thyroid cancer is a relatively rare cancer with an unusual incidence pattern with respect to age and sex. Throughout most of the world the age-standardize incidence rates are about 2.5-3 times higher in females than males. In females, thyroid cancer incidence rises steadily throughout childhood and adulthood, and peaks in the reproductive years. Between ages 15-39, thyroid cancer incidence ranks among the top five for all cancers in females. The strongest risk factor for thyroid cancer identified to date is radiation exposure. However, given the low prevalence of therapeutic and diagnostic radiation exposure, it is crucial that etiology research shift to more prevalent exposures. In recent years, epidemiologic studies of thyroid cancer have focused on reproductive and hormonal factors which might explain the dramatic female excess. For this reason, we conducted a case-control study of 108 young women diagnosed with thyroid cancer. We have extended case identification and now have a sample of 292 matched case-control pairs of females of reproductive age. This represents the largest such sample to date of females of reproductive age. We also collected data from 293 (153 case and 140 control) mothers of these subjects regarding their own medical and reproductive history and radiation exposures of their daughters. Our overall objectives are: 1) to study the role of reproductive and hormonal exposures, and other exposures which may alter thyroid function, in the etiology of female thyroid cancer, 2) to determine agreement between mothers and daughters in reports of childhood radiation exposures, and 3) to estimate the degree of familial correlation of certain risk factors for thyroid cancer. Given the size and uniqueness of these datasets, we anticipate this research will considerably contribute to our understanding of the relevance of these exposures to thyroid cancer in females.